Grabby Aliens

Advanced aliens really are out there, and we have enough data to say roughly where they are in space and time, and when we will see or meet them.


“If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare” by Robin Hanson, Daniel Martin, Calvin McCarter, and Jonathan Paulson, September 7th, 2021.

Plot of civilisations emerging in space, over time


There are two kinds of alien civilizations. “Quiet” aliens don’t expand or change much, and then they die. We have little data on them, and so must mostly speculate, via methods like the Drake equation.

“Loud” aliens, in contrast, visibly change the volumes they control, and just keep expanding fast until they meet each other. As they should be easy to see, we can fit theories about loud aliens to our data, and say much about them, as S. Jay Olson has done in 7 related papers (1, 2, 3, 4, 5, 6, 7) since 2015.

Furthermore, we should believe that loud aliens exist, as that’s our most robust explanation for why humans have appeared so early in the history of the universe. While the current date is 13.8 billion years after the Big Bang, the average star will last over five trillion years. And the standard hard-steps model of the origin of advanced life says it is far more likely to appear at the end of the longest planet lifetimes. But if loud aliens will soon fill the universe, and prevent new advanced life from appearing, that early deadline explains human earliness.

“Grabby” aliens is our especially simple model of loud aliens, a model with only 3 free parameters, each of which we can estimate to within a factor of 4 from existing data. That standard hard steps model implies a power law (t/k)n appearance function, with two free parameters k and n, and the last parameter is the expansion speed s. We estimate:

  • Expansion speed s from fact that we don’t see loud alien volumes in our sky,
  • Power n from the history of major events in the evolution of life on Earth,
  • Constant k by assuming our date is a random sample from their appearance dates.

Using these parameter estimates, we can estimate distributions over their origin times, distances, and when we will meet or see them. While we don’t know the ratio of quiet to loud alien civilizations out there, we need this to be ten thousand to expect even one alien civilization ever in our galaxy. Alas as we are now quiet, our chance to become grabby goes as the inverse of this ratio.

3D Sim Movie of Model

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